(1) Field of the Invention
The present invention relates to four wheel drive working vehicles and more particularly to a transmission structure for driving front wheels.
(2) Description of the Prior Art
In ordinary four wheel working vehicles, the front and rear wheels are driven at substantially the same speed. In recent years, however, certain types of agricultural tractor have been in practial use which include a front wheel transmission structure for driving the front wheels in a plurality of speeds. One such transmission structure is disclosed in Japanese Patent Publication Kokai No. 61-150830.
The front wheel transmission structure disclosed therein comprises a standard gear transmission for driving the front and rear wheels at substantially the same speed, an accelerating gear transmission for driving the front wheels at a higher speed than the rear wheels, and a hydraulic clutch acting as a switching device for selecting between the standard gear transmission and the accelerating gear transmission for driving the front wheels. The front wheels are driven at the higher speed than the rear wheels to enable the tractor to make a small sharp turn with ease even on a soft ground.
According to this transmission structure, the acceleration ratio of the front wheels with respect to the rear wheels are primarily determined by the gear ratio of the accelerating gear transmission. Therefore, the gear ratio of the accelerating gear transmission must be changed in order to change the acceleration ratio. Such a change necessitates rear-rangement of components of the front wheel transmission structure and a change in the shape of a casing that houses the front wheel transmission structure. Thus, a change in the acceleration ratio in the known structure requires great trouble. When the specification of the transmission structure is changed to provide different acceleration ratios, the change in the specification results in a great increase in manufacturing cost.